Multicoupling system with a distribution line



March 19, 1968 s, PARKER 3,374,446

MULTICOUPLING SYSTEM WITH A DISTRIBUTION LINE Filed Dec. 29, 1964 3 Sheets-Sheet 1 FIG. I

F/ 3 INVENTOR.

5AM E PARKER BY (ZJ u. 4 4 a? March 19, 1968 s. E. PARKER 3,374,446

MULTICOUPLING SYSTEM WITH A DISTRIBUTION LINE Filed Dec. 29, 1964 3 Sheets-Sheet 2 014/ BAA/D D-L/Nf i X, 3 a XI 1; 7 W m 'mn- -L Z X2 X5 X2 L F/ 6 7 p INVENTOR.

634M E. PAR/(El? March 19, 1968 s. E. PARKER MULTICOUPLING SYSTEM WITH A DISTRIBUTION LINE Filed Dec.

3 Sheets-Sheet 5 Val.

United States Patent 3,374,446 MULTROUPLING SYSTEM WITH A EHSTRIBUTION LINE Sam E. Parker, San Diego, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Dec. 29, 1964, Ser. No. 422,093 3 (Ilaims. (Cl. 333-8) ABSTRAQT OF THE DECLGSURE Many transmitters and receivers must be coupled to a single antenna as, for example, aboard a battleship where space is limited. The classic ladder network with parallel and shunt arms are connected at one end to the antenna. The impedances of the L and C elements of the ladder are selected so that the characteristic impedance of the ladder matches the impedance of the load. The transmitters and receivers are then coupled into the impedances elements in either the parallel or shunt arms without upsetting the match and with zero on minimum coupling between the transmitters and receivers.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a high frequency distribution line and is particularly directed to a line peculiarly adapted for coupling a plurality of variously tuned transmitters and receivers to a single broadband antenna.

Multicoupling systems heretofore have usually employed hybrid transformers or, alternatively, networks that transmit certain frequencies while rejecting others. Systems employing bridged-T networks, factional-X complementary filters, and highly selective tuned circuits have come into being. More recently it has come into vogue to employ a system of tuned coupled circuits connected in series, in parallel or in a series-parallel combination. Considerable effort has been made in transforming load impedances and is compensating for effects of the coupling configuration.

The object of this invention is to provide improved means for efiicient transmissions between a single antenna and a plurality of voltage sources or receivers with minimum inter-action between the coupled components.

The object of this invention is attained by a ladder network comprising a plurality of cascaded sections connected at one end to a broadband load such as an antenna. Each of the network sections comprise a shunt leg of impedance X and full series arms each having an impedance of X The reactances of impedances X and X are of opposite sign. A plurality of transmitters or receivers are coupled, respectively, to the series arms and/or to the shunt arms, each transmitter-receiver being sharply tuned to its operating frequency.

When coupling to a series element, the effective series impedance looking away from the load should be low at the operating frequency and, conversely, when a shunt coupling element is employed, this impedance should be relatively high. Looking toward the load, the network should not introduce unfavorable transformations of the common load impedance. Means for designing eflicient transmission networks for these purposes are known from classical image-parameter theory and from modern network synthesis techniques.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the preferred embodiments described in the following specification and shown in the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of one distribution line with specific connected series and shunt coupling elements;

FIG. 2 is a block diagram of the equivalent distribution line of this invention commonly called a ladder network;

FIG. 3 is a block diagram of iterative T-sections of the distribution network of this invention;

FIG. 4 is a schematic circuit diagram of a representative ladder network consisting of band-pass Tsections;

FIG. 5 is a block diagram of two distribution lines connected to a single load and embodying this invention;

FIG. 6 shows the reactance-frequency characteristics of the distribution line of FIG. 5, assuming that two band-pass structures are utilized.

FIG. 7 is a schematic circuit diagram of a low pass line; and

FIG. 8 is a schematic circuit diagram of a specific multiple coupling to common load of this invention.

A specific embodiment of this invention will be referred to in FIG. 1 before proceeding to the more general considerations of the invention. As indicated in FIG. 1 the multicoupling system consists of a multiplicity of highly selective networks interconnected with an antenna by means of a distribution line. The distribution line may take the form of a low pass filter as in FIG. 1, although a high-pass or a bandpass ladder network may be employed. The load 10 shown at the right hand of the line may include an antenna which presents a load impedance, Z Inductors 11 and 12 are connected in series in the line while the capacitance 13 is connected in shunt across the line. Tank circuits 14 and 15 are inductively coupled to the series inductors 12 and 11 and are sharply tuned by parallel tuning condensers to their respective operating frequencies. The shunt branch 13 is preferably made up of two series condensers giving capacitance values that are suitable for capacitive coupling. Inductive coupling is suitable for use with both the series and the shunt elements of the ladder network but capacitive coupling is generally used with shunt elements that are normally grounded. For simplicity, single mesh couplers are shown at 14, 15 and 16 in FIG. 1 for coupling signal voltages e (2 and 2 of transmitters or receivers to antenna load 10 but multi-mesh couplers may be used for greater selectivity, as required.

A general form of distribution line of this invention is illustrated in PEG. 2. The line is terminated in the antenna or common load impedance Z on one end and is short-circuited at the other end. The line would be open-circuited, of course, if the left hand element were a shunt arm such as element X The load impedance according to this invention is not adversely transformed by the line and in addition a favorable distribution of zeros and poles are provided in series or in shunt with the excitation when used with transmitters, or with responses when used with receivers. These conditions are explained with reference to FIG. 3 and FIG. 4.

In FIG. 3 the line has a characteristic impedance, Z that is primarily resistive, R and approximates the normalized value of the complex load impedance, Z For purposes of illustration the ladder network in FIG. 3 is shown as several symmetrical T-sections in tandem. These T-sections can be designed to provide an impedance of approximately R looking toward the load at each midseries point such as 1-1', or 11-11, assuming that 2;, equals R According to another highly useful property of the distribution line of this invention the network looking toward the left can present a relatively small value of reactance X in series with each mid-series arm at its frequency of excitation. i

Let it be assumed that terminals 1-1' are short-circuit- The corresponding reactance function with two T-sections in tandem is:

Normalizing to an impedance level of one ohm, for simplicity,

X X =1 (3) Then replacing l/X by X in Equation 1 yields X Xl(X1 -3 and in like manner Equation 2 may be expressed in terms of X in the following form:

X1(X, -6X +8) of X occur when X equals 0, /2 and :2; and poles occur when X equals :765, :1.848, and infinity. From classical filter theory, free transmission occurs toward the load when Having normalized to 1 ohm, it is apparent that this situation occurs in FIG. 3 when the series reactance arms X have values between 2 and +2. It follows, therefore, that certain zeroes of X and X exist in the normal pass band of the tandem filter sections.

According to this invention the band pass form of distribution line can be constructed for coupling a large number of transmitters or receivers to a common load. In FIG. 4 the distribution line includes two constant-K band pass T-sections which are short-circuited at the lefthand terminal and are connected at the right hand terminals to a load. The shunt arms of the T sections comprise inductance L and capacitance C The series arms of the T, in series in the distribution line, comprise series inductance L /2 and capacitance 2C Adjacent arms of the T-sections may be combined to form a single capacitor C as shown. Energy may be supplied to or taken from the system by means of mutual inductive coupling at each of the inductors, L 2 that comprise the series arms. It is evident now that a zero of X and of X occurs when e ri- 2:

t Llofmcz (8) If Equation 7 is rewritten in the form,

2 E (a. w (9) the resulting expression is helpful in analyzing the operating band of a multicoupling system of the type disclosed in this invention. This is especially true if, in addition to normalizing the impedance level to 1 ohm, the frequency range is investigated in the vicinity of w when given the value of 1 radian per second. Under these conditions Equation 9 simplifies to 11 1( Assuming that two T sections are employed, the poles of X occur when X becomes i /2 2 or $0.765. It can be shown that X (nearest the load) cannot be allowed to approach these values too closely else there will be serious degradation in performance.

A comparison of the poles of X and X in Equations 4 and 5 shows that the former are separated much further from w As the point of coupling moves closer to the shorted end of the distribution line, therefore, operation is possible over a progressively wider band of frequencies.

According to another and important embodiment of multicoupling of this invention, low pass band and high pass band transmission lines can be connected in multiple to a single load, as illustrated in FIG. 5. The shunt reactances X and X are parallel resonant circuits, and the series reactances X and X are series resonant circuits, as shown in FIG. 4. In these constant-K bandpass filters the series and shunt arms give resonance and anti-resonance respectively at the geometric means frequency in the pass band. Since the shunt reactance functions X and X in FIG. 5 have poles both above and below the respective operating bands of the line sections, each section can serve at the shunt arm for the other section, in a manner suggested in US. Patent 1,557,230 issued to Otto I. Zobel on Oct. 13, 1925. FIG. 6 shows variations of the series reactance arms X and X as a function of frequency of the elements of FIG. 5 together with the location of the zeros and poles relative to the low and the high operating ibands. Inasmuch as the distribution lines in FIG. 5 are operating with terminating arms that are somewhat different from those in FIG. 4, the zeros and the poles of X occur at different values of the series arms.

It has been shown that the networks in FIG. 5 may be excited by means of inductive coupling to the various series arms, X and X provided excitation occurs at frequencies in the vicinity of the zeros of X and X In the case of the constant-K bandpass networks shown here certain poles of X and X practically coincide with certain poles of the alternate mid-shunt 2X and 2X respectively.

In addition to the bandpass configuration, it is also possible to utilize low pass, high pass, and band stop sections in forming distribution networks. A consideration of the distribution of zeros and poles of the reactance function will show that certain advantages may result for specific applications. As a simple example, consider the low pass network in FIG. 7. The reactance function X is Assuming that the structure is constant-K, the transmission band extends from zero frequency to the cut-off frequency determined by the relationship Equation 11 shows that a zero of X occurs when X +3X =O (l3) and a pole exists when Since both of these critical frequencies occur in the transmission band of the low pass network in FIG. 7, it is apparent that excitation can be applied by inductive coupling to X in arm ab in the vicinity of the zero of X and by capacitive coupling to X in arm b-c in the vicinity of the pole of X Similar configurations using high pass and band stop network sections are possible in various combinations.

It is possible to design a distribution system in which the positions of the zeros or poles of the reactance function are adjusted during the course of operation. This situation is illustrated in FIG. 8 where excitations is indicated by capacitive coupling at two dilferent operating frequencies f and f A high-Q resonant circuit is coupled to each series inductor L Each of these circuits would be tuned to resonance at the operating frequency of the source that is coupled to the shunt path on the side of the load impedance. Thus, L and C would be tuned to resonance at the frequency f of the energy applied by means of coupling capacitor C In this manner, a very large impedance is reflected into the series arm L at i and, in view of the high-Q of the coupled resonant circuit, relatively little impedance is reflected into this series arm at operating frequency f shown at the left hand end of the network. The shunt capacitors such as C and C in FIGS. 1 and 8 are ganged in a manner that permits the coupling capacitor (C to be varied Without significantly changing the total shunt capacity C This arrangement afiords special advantages with regard to the number of circuits that can be accommodated. This results from the fact that the coupled resonant circuits, such as L -C elfectively provide isolation of the section of distribution line that is unused with each source of excitation.

The system of this invention may include a wide variety of distribution lines composed of various combinations of T or 11' sections in tandem and may include coupling units having from 1 to n meshes coupled inductively or capacitively to the line. Moreover, sections of distribution line having complementary transmission bands may be connected in parallel with a single antenna or other load.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In combination in a system for coupling a single complex load to a plurality of transmitters or receivers tuned to different frequencies;

an LC ladder network comprising a plurality of sections connected in tandem, said complex load being connected across one end of said ladder network, said network having a characteristic impedance Z that is substantially resistive and approximates the value of said load,

each of said sections comprising a shunt leg of imres zi X3 and series arms each of impedance X impedances X and X being of opposite signs, and X being inversely proportional to X and differently tuned circuits coupled, respectively, to said impedances X and X at frequencies for which the series or shunt reactances looking away from the load at each coupling point are, respectively, very small and very large, relative to the load reactance.

2. In combination a complex broadband load;

a distribution line connected at one end to said load,

said distribution line comprising a series of tandem constant-K filter sections each filter section comprising a T-network having a shunt arm of. impedance X and two series arms of impedance X 2,

the impedance of each section, looking away from said load, being zero and the impedance of each section, looking toward said load, being equal to the characteristic impedance of said line presented to said load,

said shunt arms each comprising anti-resonant circuits including inductance C and L in parallel, and each series arm comprising series resonant inductance L 2 and capacitive reactance 2C and means for inductively coupling an external frequency utilization circuit to each of said series inductance L /2.

3. In combination:

a complex broadband load,

a distribution line coupled to and providing free transmission toward said complex load, said line comprising a series of cascaded T sections,

means for coupling energy into or receiving energy from various shunt arms of said distribution line, and

means for coupling a tuned, high-Q resonant LC circuit to each of the series arms that lie on that side of each coupling point of the T section which is farthest from theload, said resonant circuit being sharply tuned to the frequency of excitation of the associated shunt arm toward the load, thereby providing a high impedance which effectively isolates a portion of the distribution line in the immediate vicinity of said excitation frequency.

References Cited UNITED STATES PATENTS 2,029,014 1/ 1936 Bode 333- FOREIGN PATENTS 106,733 2/ 1939 Australia.

OTHER REFERENCES Van Valkenburg: Network Analysis Prentice-Hall, New Jersey, 1955, p. 337 relied on.

HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner. 

